1. Field of the Invention.
The invention relates to an apparatus and a method for producing an object using stereolithography.
2. Description of Related Art
A stereolithography apparatus of this kind is disclosed in German Patent 41 34 265. The wiper disclosed therein may have an anti adhesive coating or may be formed of a material having low adhesion properties. If such a wiper has low adhesive forces, there is a risk that the contact between the wiper and the liquid resin material is locally broken during the wiping operation. This may cause the so-called dewetting. However, if the wiper is formed of a material having a great surface tension, irregular coatings are experienced, because the resin material rises again at the trailing side of the wiper.
Various coating methods are known for the stereolithography. According to the EP 0 171 069 a liquid layer of the light-curable resin is applied by lowering the object by an amount exceeding the desired layer thickness in a bath and subsequent rising the object to an elevation corresponding to the predetermined layer thickness below the surface of the bath. In EP 0 250 121 it is disclosed to apply the material for one layer from above using an inlet, whereas in WO91/12120 a spray tube is used. With the mentioned applying methods the adjustment speed of a desired layer thickness is not optimized.
In order to attain the layer thickness in a fast manner applying methods using a wiper are known. In the simplest case described in DE-C-41 34 265 the support carrying the object to be formed is lowered in a bath of liquid, light-curable resin material by an amount corresponding to the desired layer thickness, whereby uncured material flows over the previously cured layer from the edge thereof. In order to increase the speed of attaining the desired layer thickness with the wiper the material is spread over the previously cured layer. The applying operation may also be expedited by lowering the support in the bath by an extent greater than the predetermined layer thickness and subsequent spreading using the wiper. However, these methods are disavantageous in that the produced layer thickness is considerably greater than the desired layer thickness. A solution for this problem known to the applicant consists in collecting the amount of resin which is displaced by lowering the support in the bath and continuously reapplying the same to a place directly preceding the wiper during the wiping operation by using a continuously operating low-pressure pump, thereby obtaining a quasi-continuous application. A further known variant of an applying method is the so-called coater-channel-method wherein the amount of resin displaced by the lowering operation is pumped all at once in a discrete pumping operation into a coating channel located in front of the wiper with respect to the wiping or travelling direction thereof and delivered therefrom to a location directly upstream of the wiper through an outlet provided at the coating channel. The delivery of the resin amount from the coating channel varies exponentially over the surface to be coated as a function of the instantaneous filling level in the coating channel, i.e. the less resin is present in the coating channel, the less is delivered. This produces a layer thickness which decreases with the coating distance.
A general problem encountered when using the known wiper is the interaction of the wiper with the resin or the plastic material used which causes the above-mentioned "dewetting". Further, as shown in the FIGS. 8 and 9, the flow around the wiper 99 within "closed volumes" 100, i.e. regions of liquid resin encircled by solidified resin 101, produces a superelevation of the bath surface which causes variations in dimension of the object, in particular of the object within the "closed volume" (CV+, CV- effects). A further disadvantageous effect of the known coating method using a wiper is the nose 102 produced during the coating operation at the interface between the previously solidified layer and uncured resin, as shown in FIG. 10. Further, concave or convex surfaces may be produced by the flow around the wiper and cause dimensional inexactness of the object.